The VITAL: Effects on Bone Structure and Architecture is an ancillary study of the parent VITAL. This study has enrolled a sub-cohort of 771 VITAL participants at the NIH-sponsored Harvard Catalyst Clinical and Translational Science Center (CTSC). The following measurements will be performed at baseline and 2 years post-randomization to determine whether high-dose vitamin D supplementation vs. placebo: 1) produces small increases or reduces bone loss in spine, hip, and total body areal bone density as assessed by dual x-ray absorptiometry (DXA); 2) reduces bone turnover as assessed by biomarkers of bone resorption and formation; 3) improves (a) volumetric bone mineral density (vBMD) and measures of bone structure as assessed by peripheral quantitative computed tomography (pQCT) and (b) bone microarchitecture as assessed by high resolution pQCT (HR-pQCT) at the distal radius and tibia as well as trabecular bone score (TBS) at the spine; and 4) results in changes in body composition as assessed by DXA. Parallel assessments of the effects of omega-3 fatty acids will be performed. This study may elucidate the mechanisms through which high-dose vitamin D may prevent age-related fractures and provide new insights into the role of vitamin D on skeletal health and body composition.